This invention relates generally to systems and devices for controlling the ratio of air and fuel supplied to an internal combustion engine. More specifically, this invention relates to an air inflow control system particularly for use with diesel engines to schedule the air-fuel ratio in accordance with fuel flow to minimize exhaust pollutants.
In a diesel engine system, the engine is controlled by selecting the flow of fuel supplied to the engine in accordance with desired or required engine operating conditions, such as speed and/or load, wherein this fuel is mixed with air for combustion within engine cylinders to provide a usable power output. The air inflow to the engine, however, typically is substantially unregulated relative to the fuel flow but instead constitutes a relatively high air flow chosen to provide a desired air-fuel ratio at relatively high speed and/or high load engine operation. As a result, the ratio of air and fuel supplied to the engine may vary widely over a range of engine operating conditions, with excess air being supplied to the engine especially during relativeIy low speed or part-load operation. The amount of this excess air can be particularly significant when the engine system includes a supercharging device, such as an engine-driven air pump or an exhaust gas driven turbocharger to provide a relatively high mass flow of air to the engine.
In a diesel engine, it is known that fluctuations in the ratio of air and fuel supplied to the engine have a direct and adverse impact upon the level of toxic compounds in the engine exhaust gases. More particularly, substantial air inflow exceeding the amount necessary for complete and relatively smoke-free fuel combustion can alter combustion temperatures resulting in a substantial and undesirable increase in the generation of toxic oxides of nitrogen as combustion by-products. However, a constant air-fuel ratio for all engine operating conditions does not necessarily result in minimizing toxic emissions, whereby some relatively minor adjustment in the air-fuel ratio may be desirable.
A variety of control systems and devices have been proposed to control noxious emissions from internal combustion engines by regulating engine air inflow. For example, exhaust gas recirculation systems are well known wherein a recirculation valve responsive to engine operating conditions diverts a metered portion of substantially inert engine exhaust gases to the engine air intake thereby reducing and regulating the mass flow of air supplied to the engine. However, exhaust gas recirculation systems are accompanied by a number of disadvantages, such as contamination of the recirculation valve and/or the engine with carbon, moisture, and other contaminants present in the exhaust gases.
A variety of other control systems have been proposed for restricting or otherwise regulating the flow of air to an engine, thereby achieving at least some degree of control over engine air-fuel ratio. However, such systems have been responsive to a variety of parameters, such as barometric pressure, exhaust gas temperature, exhaust gas oxygen level and the like, wherein these parameters are not directly indicative of engine fuel flow and thus do not permit accurate control of the air-fuel ratio in a diesel engine as the fuel flow is varied throughout a range of engine operating conditions. Moreover, these systems normally control air flow by use of a throttle valve which results in an undesirable parasitic load on a supercharging device. Alternately, such systems have been designed to control air flow over a narrow range of engine operating conditions or have not accommodated accurate scheduling of the air-fuel ratio, whereby optimization of air-fuel ratio for minimum toxic emissions has not been achieved.
There exists, therefore, a significant need for an improved air inflow control system responsive directly to an operator-selected fuel flow and thus applicable for use with diesel engines to accurately control the ratio of air and fuel throughout a broad range of engine operating conditions. Moreover, there exists a need for such a control system including the capability to schedule the engine air-fuel ratio for minimum toxic exhaust emissions. The present invention fulfills this need and provides further related advantages.